Missing Link Between Planets and Stars Found

Brown
dwarfs are the oddballs of the cosmos, more massive than planets but not heavy
enough to generate the thermonuclear fusion that powers real stars. Now
astronomers have found the coldest brown dwarf to date.

The cold
brown dwarf floats freely in space, not bound to a star. Its mass is somewhere
between 15 and 30 times that of Jupiter. And it is about 660 degrees Fahrenheit
(350 Celsius), cooler than any other known object in its class.

The surface
of the sun is about 11,000 degrees F (6,000 degrees C). The temperature at the top of Jupiter's clouds is about -230 degrees F
(-145 degrees C), though at its core the mercury soars to 43,000 degrees F
(24,000 degrees C).

The
brown dwarf, named CFBDS J005910.83-011401.3, is about 40 light-years from our
solar system. It was found by an international team using the Canada-France-Hawaii Telescope and Gemini North Telescope, both located in Hawaii, and the a
European Southern Observatory telescope in Chile.

Oddballs

The
mass of brown dwarfs is usually less than 70 Jupiter masses. In contrast to a
star like our sun, which spends most of its lifetime burning hydrogen and
keeping a constant internal temperature, a brown dwarf spends its lifetime
getting colder and colder.

The
first
brown dwarfs were detected in 1995. Since then, they have been found to
share common properties with giant planets, while differences remain. For
example, clouds of dust and aerosols, as well as large amounts of methane, were
detected in the atmospheres of the coolest brown dwarfs, just as in the
atmosphere of Jupiter and Saturn.

However,
there were still two major differences. In the brown dwarf atmospheres, water
is always in gaseous state, while it condenses into water ice in giant planets;
and ammonia has never been detected in the brown dwarf, while it is a major
component from Jupiter's atmosphere.

Cooler still

The
newly-discovered brown dwarf looks much more like a giant
planet than the known classes of brown dwarfs, both because of its low
temperature and because of the presence of ammonia.

To
date, two classes of brown dwarfs have been known: the L dwarfs (temperature of
2,100 to 3,600 Fahrenheit (1,200-2,000 C), which have clouds of dust and
aerosols in their high atmosphere, and the T dwarfs (temperature lower than
2,100 F), which have a very different spectrum because of methane forming in
their atmosphere.

Because
it contains ammonia and has a much lower temperature than do L and T dwarfs,
CFBDS0059 might be the protoype of a new class of brown dwarfs to be called the
Y dwarfs, the researchers propose. This new class would become the coldest
stellar objects, hence the missing link toward giant planets.

Almost a planet

This
discovery also has important implications in the study of extrasolar planets.

The
atmosphere
of brown dwarfs looks very much like that of giant planets, therefore the
same models are used to reproduce their physical conditions. Such modeling
requires to be constrained with observations. Observing the atmospheres of
extrasolar planets is indeed very hard because the light from the planets is
embedded in the much stronger light from their parent star. Because brown
dwarfs are sometimes isolated bodies, with no stars nearby, they are much
easier to observe.

So
looking to brown
dwarfs with a temperature close to that of the giant planets will help in
constraining the models of extrasolar planets' atmospheres, the researchers
said.

The
discovery, led by researchers at Observatoire de Grenoble in France, the Canada-France-Hawaii Telescope, the University of Ottawa and other institutions will
be detailed in the journal Astronomy & Astrophysics.